If you're referring to ones that say "Getaway Sale" at the bottom, that confused my friend, too; they're random. 10 different games are headlining 10 days of sales, and so they're the ones featured on the set of trading cards used to earn the summer sale badge. Those cards are earned for participating in the sale or crafting game badges. You don't get the Tomb Raider card for owning that game, or the Skyrim card for owning that one, but you do get some of their own set of cards for playing them (like you did with The Witcher 2).

All you do to earn your free card drops is have the game running, so some people just alt-tab at the main menu. I'm not sure whether it's a small chance you could get one every minute, or if it's every X amount of time, but it usually takes up to 2-3 hours, depending on how many cards are in the set. You get half the total number in the set, rounded up, but that might include a duplicate or two. And so, it is mandatory to either trade with friends or buy/sell on the market in order to complete a set and craft a game badge. Crafting a badge gets you XP to level up your Steam account, a random emoticon and profile background image associated with that game, and for now, a Getaway card. Before the sale, that last thing was a random coupon.

Basically, it's a pretty genius hybrid of gamification and "collect 'em all" syndrome to get people more involved with Steam and the games available there. Kind of like how in previous sales they encouraged buying games you don't necessarily want, in order to achieve some objective and get a reward of not very high value, except the trading cards thing is happening ALL THE TIME now, with sets constantly being added to more games (new sets will probably eventually be added to games that already have them, too)... Meanwhile Valve takes 5% of each of the zillions of transactions going on in the community market, heh. Developers get 10%.